Field Transmitter: Temperature sensor

Field Transmitter: Temperature sensor Solutions from Texas Instruments

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Design Considerations

Field Transmitter: Temperature Sensor

Temperature Sensor solutions are used to measure temperature and have a wide range of uses in industrial, commercial applications. Temperature measurements are made using RTD (Resistance Temperature Devices), Thermistor or Thermocouple.

RTD and Thermistor utilize the property of Change in Resistance according to temperature to measure voltage drop across the element and calculate the temperature. A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference, known as Seebeck effect. Ambient temperature is added to this calculated temperature difference to measure actual temperature. The process of measurement of Ambient Temperature is known as cold junction compensation.

Temperature transmitters are of two types. The conventional Field Transmitters working as a part of Industrial control loop that are line or loop powered and the Wireless or Handheld temperature sensors that are battery powered.

Excitation Technique: RTDs and Thermistors being passive elements need a current excitation source to make measurements. Highly stable and accurately matched current sources with low drift are critical to making 3 wire RTD measurements. TI’s temperature sensor front-ends have high precision on-chip current sources. Accurate measurement of the ambient temperature for Cold Junction compensation in Thermocouple measurements can be made using the high accuracy on chip temperature sensor. Thus the TI analog front-end for temperature measurement is a single chip solution that provides all aspects of signal conditioning needed for RTD, Thermistor and Thermocouple measurements.

Signal Conditioning/Signal Acquisition and Processing: The output range of the temperature sensor is very small and thus the signal needs to be amplified before processing to prevent introduction of errors. The signal chain has to handle the small signal accurately in presence of noise. TI’s temperature sensor front-ends have an input multiplexer, high impedance PGA and high resolution sigma-delta modulator all integrated into a single chip. The low noise, chopper stabilized PGA provides the versatility of scaling the gain as needed in accordance with how small or large the output from the sensor is. High end products also have integrated features for system diagnostics and fault monitoring. A further level of system calibration could be implemented in the MCU.

Interface and Communication: Traditional analog (4 - 20mA) interface remains the popular choice for industrial control and sensor applications. The other popular protocols include HART, Profibus and IO-Link. TI's IO-Link interface products have integrated regulators and diagnostic outputs. In addition, wireless options based on IEEE 802.15.4 protocols are becoming more prevalent. TI is committed to provide solutions for both traditional and emerging industrial interfaces.

Power Management: The Field Transmitter can be powered in one of three ways. Line powered transmitters are commonly powered by voltage rails of 12V, or 24V. Loop powered transmitters are powered by the 4-20 mA loop. Such transmitters require extremely low power architectures as the entire solution has to be powered off the loop. TI provides high efficiency Step Down converters with low quiescent current and low output ripple appropriate for Line and Loop powered transmitters. Battery powered transmitters powered can be designed using TI's low power Buck and Buck-Boost converters. The DC/DC buck converters offer over 95% efficiency over a wide battery voltage range, even with input voltage down to 1.8 volts extending battery life. Special Buck-Boost converters generate a stable required output voltage and supply constant current for over- and under-input voltage conditions and support various battery configurations.

Microcontroller: The Hercules™ Safety MCUs offer an ARM Cortex-R4F based solution and are certified suitable for use in systems that need to achieve IEC61508 SIL-3 safety levels. These MCUs also offer integrated floating point, 12 bit ADCs, motor-control-specific PWMs and encoder inputs via its flexible HET Timer co-processor. Hercules Safety MCUs can also be used to implement scalar and vector-control techniques and support a range of performance requirements.

Application Notes (8)

Title Abstract Type Size (KB) Date Views
HTM 8 KB 09 Oct 2011 6614
HTM 8 KB 13 Jan 2011 2672
HTM 8 KB 08 Jan 2007 1066
PDF 177 KB 10 Oct 2005 1608
HTM 9 KB 28 Jun 2005 826
HTM 8 KB 08 Mar 2005 1067
HTM 8 KB 15 Nov 2004 1442
HTM 8 KB 22 Sep 2000 2513
    

Reference Designs

Selection and Solution Guides

Selection Guides (5)

Title Abstract Type Size (KB) Date Views
PDF 8.94 MB 11 Mar 2013 28508
PDF 273 KB 16 Jul 2012 1775
PDF 302 KB 28 Mar 2012 3440
PDF 916 KB 22 Mar 2011 782
PDF 175 KB 11 Jun 2010 2251

Solution Guides (5)

Title Abstract Type Size (KB) Date Views
PDF 4.07 MB 12 Nov 2013 5000
PDF 896 KB 12 Nov 2013 7403
PDF 1.08 MB 31 Oct 2013 7533
PDF 599 KB 30 Oct 2013 3651
PDF 2.13 MB 30 Oct 2013 3367

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